Contact Time of Droplet Impact on Inclined Ridged Superhydrophobic Surfaces

Superhydrophobic surfaces decorated with macrostructures have attracted extensive attention due to their excellent performance of reducing the contact time of impacting droplets. In many practical applications, the surface is not perpendicular to the droplet impact direction, but the impacting dynamics in such scenarios still remain mysterious. Here, we experimentally investigate the dynamics of droplet impact on inclined ridged superhydrophobic surfaces and reveal the effect of We(n) (the normal Weber number) and alpha (the inclination angle) on the contact time tau. As We(n) increases, tau first decreases rapidly until a platform is reached; if We(n) continues to increase, tau further reduces to a lower platform, indicating a three-stage variation of tau in low, middle, and high We(n) regions. In the middle and high We(n) regions, the contact time is reduced by 30 and 50%, respectively, and is dominated by droplet spreading/retraction in the tangential and lateral directions, respectively. A quantitative analysis demonstrates that tau in the middle and high We(n) regions is independent of We(n) and alpha, while the range of middle and high We(n) regions is related to alpha. When alpha < 30 degrees, increasing alpha narrows the middle We(n) region and enlarges the high We(n) region; when alpha >= 30 degrees, the two We(n) regions remain unchanged. In addition, droplet sliding is hindered by the friction and is affected by the droplet morphology in the high We(n) region. Overall, the synergistic effect of the surface inclination and macrostructures effectively promotes the detachment of impacting droplets on superhydrophobic surfaces, which provides guidance for applications of superhydrophobic surfaces.